All the gold on the earth is thought to have come from asteroids. All the gold that was present when the Earth was formed sank to the core a long time ago.[1]

The moon is thought contain Helium 3 which can be used for fusion reactors.[2]

[1]http://www.sciencedaily.com/releases/2011/09/110907132044.ht...

[2]http://www.esa.int/Our_Activities/Preparing_for_the_Future/S...